Implement Dart A2UI agent SDK library.

Use blueprint: https://github.com/a2ui-project/a2ui/blob/main/blueprints/modules/a2ui_agent.blueprint.md This doc can help with order of implementation: go/a2ui-v1_0-implementation-plan

Library name: a2ui_agent

• 0.9
  • json_schema_builder: https://github.com/flutter/genui/pull/1029
  • core: https://github.com/a2ui-project/a2ui/issues/2373
    • main PR: https://github.com/a2ui-project/a2ui/pull/2439
    • follow up for https://github.com/a2ui-project/a2ui/pull/2439#discussion_r3928080732: https://github.com/a2ui-project/a2ui/pull/2538
  • agent:
    • Prototype for API and tests: https://github.com/a2ui-project/a2ui/pull/2408
    • Prepare tests: https://github.com/a2ui-project/a2ui/pull/2633
• 1.0 (blocked by not-readiness of 1.0 blueprint)
  • core
  • agent